The present invention relates to apparatus and a method for removing gaseous pollutants from the flue gas of solid fuel fired steam generators and more particularly to apparatus and a method for removing elemental mercury and mercury compounds from the flue gases from coal fired boilers.
The use of activated carbon and carbonaceous particles for the adsorption of pollutant gases, such as mercury vapor has been successfully demonstrated in various applications, such as municipal waste incineration. However, there are significant differences in the concentration of mercury from waste incinerators compared to coal-fired power plants with the concentration from the coal-fired power plants being anywhere from 10 to 100 times lower. Also, the mercury from waste incinerators is usually in the form of mercury chloride whereas a larger percentage of mercury from coal-fired power plants is usually in the form of elemental mercury. Both of these differences make it more difficult to remove the mercury from the flue gas from a coal-fired power plant.
The efficiency of the sorbent is limited by its surface area to mass ratio. A relatively large particle has a low available surface area/mass ratio that limits the adsorption of pollutant gas. Using a carbonaceous sorbent with mean particle size of about 5 microns with a maximum size of about 10 microns would improve adsorption efficiency, but storage, handling, transport and dispersion of these small particles is extremely difficult.
In conventional methods, the sorbent particles are injected in the flue gas duct upstream of particulate removal device such as baghouses and electrostatic precipitators and downstream of air heaters. The particle removal devices then collect the sorbent with the adsorbed the pollutant gases.
U.S. Pat. No. 7,780,765 issued Aug. 24, 2010 describes the injection of activated carbon into flue gas by using injected compressed air. This reduces mercury emissions in flue gases, but can become quite costly due to the need for equipment to provide the compressed air and the power the equipment uses.
It is also known in the art that vapor phase mercury in the flue gas emerging from the high temperature boiler is in the form of elemental mercury. Oxidation of elemental mercury to oxidized mercury (Hg2+) is beneficial to mercury control since it can be removed more easily by carbonaceous sorbent. Similarly, combination of elemental mercury with halogens, results in a compound that has greater affinity for the sorbent.
Currently, there is a need for a system that efficiently and economically removes gaseous pollutants, such as elemental mercury and mercury compounds from combustion flue gases of solid fueled boilers.